The present invention relates to exhaust systems, in particular to an exhaust system wherein scavenging occurs, especially in combination with silencing.
In internal combustion engines it is quite common for two or more of the exhaust valves of various cylinders to be open at the same time. Normally, the exhaust gas pressure in the cylinders with exhaust valves open at the same time will not be the same. For example, for one cylinder the exhaust port may have just opened and the exhaust gasses within the cylinder are at a relatively high pressure, whereas simultaneously the exhaust valve for another cylinder may have been open for some time such that most of the gasses within the cylinder have escaped and the valve has started to close. If the gasses escaping from these various cylinders are all vented into the same exhaust manifold, it is quite possible, due to poor engineering, that the exhaust gasses from the cylinder where the valve has just opened may at least partially repressurize the cylinder where the valve is just about to close with exhaust gasses thereby substantially decreasing the efficiency of the engine.
Over the years various exhaust configurations have been designed in attempts to alleviate the above mentioned problem and thereby increase the efficiency of the engine. For instance, exhaust gasses from cylinders which would have exhaust valves open at the same time were vented into separate exhaust manifolds. However, higher efficiency has been obtained when, through proper design, the exhaust gasses escaping at high pressure from one cylinder are utilized to draw exhaust gasses from a cylinder at relatively low pressure before the exhaust valve of the latter closed. This operation and related operations are generally referred to as "scavenging".
In theory, the performance (as used herein--the relative horsepower at any given rate of revolution of the engine) can be improved by decreasing the amount of exhaust gasses remaining in a cylinder prior to intake of fresh fuel. It is desirable that the fuel-air mixture which is combusted in each succeeding fuel-air burn in a cylinder contain a minimal amount of the burnt fuel-air mixture of the preceeding combustion. When it is not possible to have only a given fuel-air mixture in the cylinder, it is preferred that fresh ambient air be included therewith, rather than exhausted gasses from a previous combustion. Therefore, the function of the present invention is to reduce the amount of residual combusted gasses in a cylinder for the succeeding burn as much as possible.
It is also important in exhaust systems of this type to limit the amount of noise produced by the escape of gasses from the end of the exhaust system into the ambient atmosphere. Various types of silencing devices have been developed over the years which use different techniques to deaden the noise escaping from the exhaust system utilizing same. It is possible to lower or dampen the amount of noise emitted by a vibrating or oscillating material by combining that material with a similar material which is vibrating 180.degree. out of phase with the first material such that the vibrations in the two materials in effect cancel each other. It is desirable that a silencing system utilizing such resonance dampening be available for use in conjunction with the above mentioned exhaust system and, in particular a silencer which does not substantially impair the increased efficiency produced by the exhaust system.